Gunning devices that project a material onto a target substrate for producing or repairing of refractory linings are generally known. Two widely used gunning methods for fabricating and repairing refractory linings are known as the gunnite-type and shotcrete-type gunning methods. Unlike other casting methods, these gunning methods require no framework for casting refractory linings and allow for easy application even on irregular shapes or where frameworking is difficult to construct. Accordingly, gunning methods have been widely used in fabricating and repairing refractory linings, particularly, in furnaces such as a blast furnace, hot stove, electric furnace, converter, ladle, tundish, basic oxygen furnace and reheating furnace.
In a gunnite method, also known as dry-mix method, a dry powdery material to be “gunned” is pneumatically fed through a transporting hose to a nozzle assembly where a liquid, such as water, is added to produce a wet, highly viscous gunning material with good adhesive properties. The gunning material is projected through the nozzle assembly so that the material adheres and cures on the furnace wall portion, whereby a refractory furnace lining is fabricated or repaired. The gunnite application method requires no premixing of material with the liquid and can therefore be carried out rapidly and on short notice and clean-up of equipment is minimal. An additional advantage over other methods of fabricating or repairing furnace linings include not having to use a lining mold, thereby enabling cost reduction and improving working efficiency and enables the repair of both hot and cold furnace linings. However, one disadvantage of the gunnite method is that it is difficult to completely wet and thoroughly mix the material and water stream as it is transported through the application gunning lance, pipe or nozzle. This is particularly true for short (less than about 5 feet) gunning pipes. In these situations, a lack of thoroughness in mixing results in less than optimum and desirable applied mass homogeneity and density, an increase in material waste due to rebounding aggregate and poor mass adhesion and often excessive material pipe drip. Additionally, when a directional change in the flow of the gunning material is required, the material tends to exit the nozzle in a “split” non-homogenous stream where part of the stream is very dry while the other part is overly wet, a phenomenon that is independent of any attempted water control. A problem associated with an overly dry or poorly wet gunning material that is gunned onto the object target, is that a portion of the material does not adhere to the substrate and causes a loss of deflected particles (known as “rebound”) which lowers the adhesion percentage of the gunning material to the furnace wall, thus affecting the quality and durability of a refractory furnace mass. To, overcome the problems associated with the gunnite methods, shotcrete-gunning methods were developed.
Shotcrete gunning methods, also known as wet-mix method, produce refractories having a more uniform quality and better physical properties than obtained by the gunnite method and generally are used for producing high density, monolithic structures. In the shotcrete method, a gunning material is produced by mixing a dry material with liquid, such as water, in a separate mixing device prior to delivery to a gunning device. The dry powdery material is pre-wet with the liquid in a mixer and then pumped by a delivery pump through a transfer hose to a gunning device which projects the gunning material to a target using compressed air. Usually, a setting agent is added to the gunning material at the nozzle prior to the gunning material being projected onto a furnace wall structure.
The shotcrete gunning method is not without its attendant drawbacks, however, in that it is necessary to mix the dry material with the liquid in a separate vessel until a suitable consistency is obtained. Thus, a shotcrete gunning material is mixed before it is supplied by the delivery pump to a gunning device requiring additional equipment, e.g., mixer and delivery systems, and manpower, when compared with the dry-mix gunnite method. Moreover, it is important to accurately control the amount of liquid to the gunning material in the shotcrete gunning method to maintain the proper consistency. As a result, skill on the part of the shotcrete-mixer operator is required to maintain the correct amount of water for a desirable composition. If too little water is used, blocking or premature hardening of the gunning material may occur in the pump or delivery hose. Conversely, if an excessive amount of water is used, there can be separation of aggregates of coarse particles and fine powder which is contained in the gunning material to be sprayed causing uneven and poor quality refractory layers.
The conventional nozzles used in the shotcrete applications utilize a constricted nozzle tip generally made of flexible material such as polyurethane. The constricted conventional shotcrete nozzle tips are susceptible to blow-outs. Blow-outs occur when the wet-mix shotcrete aggregate material clogs the narrowing nozzle tip. Under the pressure of the wet-mix material being pumped out of the nozzle, the polyurethane nozzle tips blowup like a balloon and rupture, thus, blowing out substantial portion of the nozzle tip. Once the conventional nozzle tip is damaged by a blow-out, the shotcrete cannot be sprayed in controlled manner. The wet-mix shotcrete material will be ejected from the nozzle in uncontrolled shape and also excessive amount will be lost to dripping from the blown out nozzle tip.
In addition, the constricted shape of the conventional shotcrete nozzle tip, which is designed to control the spray pattern of the wet-mix shotcrete material and also to accelerate the velocity of the shotcrete material through Venturi effect, tends to wear through use. The worn-out nozzle tip then loses the ability to maintain proper spray pattern.
The foregoing illustrates limitations known to exist in present refractory coating methods and devices. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly an alternative apparatus for the gunning of a material is provided including the features more fully disclosed hereinafter.